For reasons still unknown to science, artificial light attracts birds, from fledgling seabirds to migrating songbirds (it does the same to moths). Once captivated, disoriented birds may crash into windows, or spend hours circling.

The 9/11 tribute is particularly problematic: dozens of 7,000-watt bulbs allow it to reach four miles into the sky—it’s visible from 60 miles away. So New York City Audubon members and volunteers take two-hour shifts underneath, scanning the beams, counting birds. Every time 1,000 birds or more are circling—or an exhausted bird falls to the ground—they alert the Municipal Art Society of New York (or MASNY, the organization behind the display), which immediately turns off the lights for 20 minutes, giving birds a chance to clear the area.

Some 9/11 survivors have also volunteered to be monitors, with Debra Kriensky, a conservation biologist, saying that they "have told me the last thing they want from this memorial, which is so meaningful and beautiful…is for there to be more death on this spot."

David Ringer, head of communication for the National Audubon Society, Tweeted videos and images of the birds last night: